As natural gas is produced from gas wells, the pressure in the formation will decrease, resulting in a reduction in gas flow rate and associated gas velocity. Before the natural drive pressure is reduced, the flow rate and velocity of produced gas may be sufficient to remove the liquids from the well with the gas. However, at some point the flow rate of gas will be insufficient to carry liquids out of the well. As a result, the liquid loading in the well will increase, and liquid will collect in the bottom of the well further reducing its output.
When production by natural reservoir pressure becomes uneconomical, artificial lift techniques can be utilized to increase well production. A number of artificial lift systems are known in the industry, including sucker rod pumps, gas lift techniques and plunger lift techniques.
A plunger lift is an artificial lift method used to de-liquefy natural gas wells and high gas-to-liquid ratio oil wells. A plunger is used to remove contaminants from productive natural gas wells, such as water (as a liquid or mist), oil, condensate and wax. FIG. 1 shows a schematic of a typical plunger system, including: a Lubricator to cushion the impact of an arriving plunger and provide safe access to the plunger; a Catcher which catches and holds the plunger in the lubricator for save removal; a controller to open and close the motor valve using time, pressure, or flow rate and provide production history for the operator; a Motor Valve pneumatic diaphragm-activated valve to start and stop the well's production based on input from the controller; a Solar Panel to provide a power source to the controller batteries; a Drip Pot to prevent downtime by trapping and preventiong condensate, water, and other contaminates from clogging the latch valves; an Arrival Sensor to signal the plunger's arrival to the controller; a Plunger stell “piston” that acts like a swab creating a seal to the tubing and lifting liquids and solids (sand, salt, coal fines, paraffin, and scale) to the surface; and a Bottom Hole Bumper Spring that sits above the seating nipple, protecting the plunger upon impact and can also hold a ball and seat to trap liquids in the tubing. The plunger cycles between the top and bottom of the well to lift fluids to the surface, as illustrated in FIG. 2. A more detailed graphic of a plunger lift system is also in FIG. 3.
The basic function of the plunge lift controller is to open and close the well shutoff valve at the optimum times, to bring up the plunger and the contaminants and maximize natural gas production. A well without a de-liquefaction technique will stop flowing or slow down and become a non-productive well, long before a properly de-liquefied well will.
Conventional plunger lift systems, which are also known as free piston systems, utilize a plunger (piston). The well is shut in and the plunger falls to the bottom of the tubing and onto a bumper spring, seating nipple or stop near the bottom of the tubing (FIG. 2, “Off Time”). After pressure in the well has built, the wellhead is opened to flow and the high pressure gas located within the well pushes the piston upward to the surface ((FIG. 2, “Lift”), thereby pushing the liquid on top of the plunger to the surface and allowing the well to produce for as long as possible (FIG. 2, “After flow”). This sequence can be repeated by closing the wellhead off and allowing the plunger to fall again to the bottom of the well while pressure in the well is allowed to rebuild.
Automatic control of plungers used in plunger lift systems is known in the art. Generally, an electronic controller can be utilized that is able to control all of the various valves required to open and close the well, monitor the position of the plunger, and if the well is equipped with a plunger catcher, catch the plunger at the surface. Such controllers may, for example, use pressure within the well, production flow rate, or travel time of the plunger in order to determine when to perform various operations. Alternatively, an electronic controller may simply operate based on a preset, timed schedule.
U.S. Pat. No. 7,681,641, for example, describes a self-adjusting process to adjust thresholds based on plunger arrival. In turn, those thresholds are used as open and close triggers that open and close the sales valve (i.e. determine how long it is shut-in or flows).
U.S. Pat. No. 7,464,753 describes the use of non-linear (fuzzy logic) to make adjustments to open and close triggers based on looking at plunger arrival time for previous cycles, with the previous cycle data stored in the micro-processor memory. This is an attempt to improve the efficiency of the self-adjust process by allowing for variable sized changes to control thresholds.
U.S. Pat. No. 6,241,014 uses dampened response and exponential response as a method to determine how much to adjust open and close triggers, with all references being made to time-based triggers.
U.S. Pat. No. 5,957,200 uses a microprocessor to evaluate tubing and casing pressures as open and close triggers.
Even though each of these patents discusses adjustments made to open and close triggers and that making such adjustments are necessary to optimize production from a plunger lift well, none of them ever describe what constitutes an optimized plunger cycle or what logic is required to achieve that optimization. It is quite possible the patents use purposefully vague language, because it was not understood what an optimized plunger cycle looks like, nor which parameters should be modified and how for optimization purposes.
This invention provides these missing components.